How important is correct spelling and grammatic for SEO?
If forum users post comments they often make spelling or grammatic mistakes. I would like to know what is the consequence for SEO? Does Google detect bad spelling and grammatic? Does Google value such content less?
Yes, Spelling and Grammar matter in SEO
It is not long past, I used to think blogging doesn't have to be perfect. Readers have to understand the message you are trying to convey. What matters is, you are trying to reach an audience. So, your writings need to be perfect in terms of SEO. Let me explain how why spelling and grammar are so important.
In response to your first query-
SEO tries to ensure the content available to human visitors. Correct grammar and spelling will make it easy for web surfers to access desired information. If you can satisfy the surfers, it is probable that the visitor will come back to you again. Visitors are the ultimate outcome of fabulous web content. That's how excellent grammar and spelling can help you win the battle in SEO. Wrong grammar makes it much harder for another person to understand. It becomes harder not only for those who are the native speaker but also for those whose primary language is different.
In response to your last two queries -
I would recommend you this Video where Matt Cutts explains what Google thinks of spelling and grammar error. The video explains, if you are too lazy to proofread your contents then they would also be lazy enough to rank your site in a higher position. Since October of 2015, Google launched an AI to monitor the pages just like the way a user sees them. They claim it helps them to ensure user experience. If you want to know what Bing thinks about this, you may probably want to read this article. Here Bing also claims to check spelling and grammar errors to ensure user experience.
I hope you get the answer.
If I am right
As far as I understood its no direct ranking signal for Google. I personally think that Google would like to take it as ranking signal but a failure free algorithm to detect correct grammar and spelling is not easy to find. Machines do make mistakes. But as users will stop reading if they see bad spelling or grammar at a website makes it a ranking signal from the backdoor as the time users use a site is a ranking signal. It shows that the content is good. Bad content will be soon clicked away.